To compare the serologic response to two different measles vaccine strains and vaccination at different ages, we conducted a randomized trial of Edmonston-Zagreb (EZ) and Moraten measles vaccines in the US. Study groups included: EZ1 (4.84 TCID50) at 6 months, EZ1 at 9 months, EZ2 (3.84 TCID50) at 9 months, Moraten (3.79 TICD50) at 9 months and Moraten (as combined measles, mumps, and rubella vaccine, MMR) at 12 months. Antibody (Ab) was measured before vaccination (prevacc) and 3 months postvacc by plaque reduction neutralization. (% with Ab>1:120 postvacc, by prevacc Ab) Vaccine group prevacc - prevacc + Total n % n % n % EZ1 - 6 mos 88 94 154 56 242 70 EZ1 - 9 mos 125 96 69 83 194 91 EZ2 - 9 mos 104 88 56 57 160 78 Moraten - 9 mos 119 95 74 72 192 86 Moraten -12 mos 120 94 16 75 136 92 Seroconversion rates for all vaccines were lower in the presence of maternal Ab. Seroconversion rates were similar in children who received high dose EZ at 9 mos and Moraten at 12 mos. While seroconversion rates were lower with Moraten at 9 mos compared with Moraten at 12 months,the differences were not statistically significant.